docs: def flag(self, diagnostic, thresh=None):
          '''
          Returns indices of diagnostic that satisfy (return True from) the
          threshold predicate. Will use class-level default threshold if
          None provided.
  
          Args:
              diagnostic (str): name of the diagnostic
              thresh (func): threshold function (boolean predicate) to apply to
              each element
          '''
          if thresh is None:
              thresh = self.defaults[diagnostic]
  
          result = self.results[diagnostic]
          if isinstance(result, pd.DataFrame):
              if diagnostic == 'CorrelationMatrix':
                  result = result.copy()
                  np.fill_diagonal(result.values, 0)
              return result.applymap(thresh).sum().nonzero()[0]
          else:
              return result.apply(thresh).nonzero()[0]